Transmitters in base stations and terminals for mobile telephony as well as transmitters for broadcast and other wireless systems all need power amplifiers (PA) to amplify the radio frequency (RF) signal to the antenna. Often, this PA needs to be very efficient to increase battery time, decrease the energy cost, and minimize cooling needs.
Traditional class B and AB PAs usually operate with constant load and constant supply voltage. The “optimal load” (Ropt) is the load that gives the highest output power under allowed operating conditions. Class B or AB operation means that the transistor current pulses approximately have the shape of half-wave rectified sine waves. The current pulses have largely the same shape at all amplitudes, and both the RF output current and the DC supply current, and hence DC power, are therefore approximately proportional to their height. This is also the case with the RF output voltage. The RF output power is proportional to the RF output current squared, which means that the DC to RF efficiency is approximately proportional to the RF output voltage amplitude. Due to this proportionality, the average efficiency for a class B amplifier, outputting a signal whose average signal level is well below the maximum (peak) level, is low compared to the efficiency at maximum output.
Load Modulation (LM) or Dynamic Load Matching (DLM) is a method to increase an amplifier's efficiency for amplitude modulated signals by having a matching network that can be varied at signal envelope speeds. By dynamically re-matching the load to the RF transistor the average RF current can be reduced. The average efficiency is increased by having the matching network transform the load resistance into a high resistance at the transistor output node at low output levels and a lower resistance equal to the optimal class B resistance Ropt at maximum output level. This lowers the RF output current at all output levels except the maximum, while producing the same RF voltage and current in the load. The RF voltage at the transistor output is at the same time increased. Reduced RF output current is translated into reduced DC current if some efficient operation class, e.g. class B, is used.
Segmentation of the amplifier into several smaller amplifiers that are individually coupled by quarter-wave transmission lines to the output increases efficiency. Efficiency maxima at several backed of amplitudes are obtained. This is because the quarter-wave lines, which have characteristic impedance of Ropt of the transistor they are connected to, transform the load resistance into a higher resistance than Ropt at the transistor outputs. This type of amplifier then has a lower average sum of RF output currents from the transistors than a conventional amplifier. The RF voltages at the active transistors' outputs are at the same time increased. The switching in and out of amplifier segments can be done either by switches or by tuneable circuits.
Real transistors often have substantial parasitic losses that reduce the achievable efficiency. The above described efficient amplifiers reduce the average output current, which minimizes loss that can be seen as effectively in series (with the load) at the transistors' output nodes. Another type of loss mechanism can be seen as effectively in shunt (coupled from node to ground) at the output node. This loss gets worse in amplifiers using the abovementioned efficiency increasing methods since they depend upon high RF voltages at the transistors. Such shunt loss is common in practical RF power transistors so the theoretical efficiency gains of the methods are often reduced in practice.
Segmented amplifiers with quarter-wave lines suffer from increased shunt loss due to the higher RF voltages at transistor outputs. The efficiency for a segmented (3 binary weighted segments) amplifier with quarter-wave lines from each amplifier segment to the load, for no shunt loss (upper trace) and loss due to a shunt resistor (lower trace) is shown in FIG. 1.
It can be noticed that the efficiency of segmented amplifiers is degraded by shunt loss, more for lower output amplitudes than for higher.
Shunt loss at the transistor output is the most detrimental loss in Load Modulation amplifiers as well. The effect is big since this loss is proportional to the squared voltage at the transistor output node, i.e. the same voltage that is increased at all output levels as an effect of the dynamic load transformation. FIG. 2 shows the optimal output voltages and currents (normalized) for LM amplifiers with no shunt loss and loss due to a finite shunt resistance.
The RF current is much higher for the transistor with shunt loss, resulting in a lower efficiency. For the lower part of the amplitude range, the load is fixed at a value about the same as the shunt resistance, and the voltage is proportional to output amplitude. The resulting efficiency for the LM amplifiers with and without shunt loss is shown in FIG. 3.
Accordingly, also the LM amplifier's efficiency is degraded by shunt loss; more for lower output amplitudes than for higher output amplitudes.